The role of the renin angiotensin system in the fetal response to hypovolemic stress will be studied in chronically instrumented fetal lambs. Angiotensin-II analog (Saralasin) and angiotensn-I converting enzyme blockers (SQ20881) will be used. Fetal arterial pressure, pH, PO2, PCO2, umbilical blood flow, and plasma renin activity will be measured before and after acute hypovolemia, with and without blockade of the renin angiotensin system.